In many types of gold balls, circular dimples are uniformly arranged on a surface thereof. When the golf ball with circular dimples is hit by a golf club, the golf ball shows a high initial speed with high elasticity, and the golf ball has reverse-rotation in the direction opposite to the flying direction, that is, backspin according to a golf club loft angle.
In the flight of the golf ball with backspin, the circular dimples cause a lift force to be generated from pressure difference between the upper and lower sides of the golf ball, so that a staying time is increased, and thus, a flying distance is increased. In addition, with respect to the air flowing along the surface of the golf ball, turbulent flow occurs in the vicinity of the circular dimples, and a departing point of a so-called boundary layer is sent backward, so that a drag coefficient is decreased, and thus, the flying distance is increased.
After the golf ball lofting at a high speed reaches a peak of the trajectory, the golf ball is dropping toward the ground due to gravity. The flight trajectory of the golf ball is mainly divided into a high speed range from the launching position to the peak and a low speed range from the peak to the landing position of the ground.
The flying characteristics of the golf ball in the flight trajectory are determined mainly by an arrangement pattern of the circular dimples and a diameter, shape, and depth of the circular dimple. Among the factors, the difference in the flying distance between the golf balls occurs dominantly due to a volume of the circular dimple which is defined by the diameter and depth of the circular dimple.
In the case of a golf ball with circular dimples having a large diameter, the lift force is easily obtained after the impact, so that the flying height of the golf ball can be rapidly increased. This ball is called a high-trajectory golf ball. However, the high-trajectory golf ball is greatly influenced by wind. When a head wind is blowing in the direction opposite to the flying direction of the golf ball, the flying height of the golf ball is too rapidly increased, so that the flying distance is decreased and the landing position is not uniform (the golf ball has bad directionality).
On the contrary, in the case of a golf ball with circular dimples having a small diameter, the lift force is not easily obtained after the impact, so that the flying height of the golf ball is low and the golf ball is early landed. In other words, the flying distance is small, and the staying time is short. This ball is called a low-trajectory golf ball. However, in comparison with the high-trajectory golf ball, the low-trajectory golf ball is not greatly influenced by wind, so that the directionality is not bad.
In addition, in the case of the circular dimples having a large diameter, the drag coefficient is large in the high speed range, but the drag coefficient is small in the low speed range. On the contrary, in the case of the circular dimples having a small diameter, the drag coefficient is small in the high speed range, but the drag coefficient is large in the low speed range.
On the other hand, as described above, the flying characteristics of the golf ball is also determined by the depth of the circular dimple. The deepest portion of the circular dimple is the central portion of the circular dimple.
Among the circular dimples with the same diameter, the deeper circular dimple has a larger volume, so that the lift force can be easily obtained. However, in the high speed range, the pressure drag is increased, so that the flying distance cannot be easily improved. Particularly, if the circular dimple is too deep, there is a problem in that the golf ball is greatly influenced by wind.
On the contrary, among the circular dimples with the same diameter, the shallower circular dimple has a smaller volume, so that a sufficient lift force cannot be obtained. Therefore, there is a problem in that the flying distance is decreased.
In addition, practically, the depth for the volume of the circular dimple as one of the important factors for the flying characteristics of the golf ball cannot be easily adjusted in production of a mould cavity for manufacturing the golf ball. In addition, several problems occur in the formation of covers of the golf ball. Therefore, there is a practical problem in that the depth of the circular dimple cannot be easily adjusted arbitrarily without consideration of the diameter of the circular dimple.
In addition, in most cases of golf balls with general circular dimples, the volume is in a range of from 350 mm3 to 500 mm3. If the diameter of the circular dimple is increased, the volume is increased; and if the diameter of the circular dimple is decreased, the volume is decreased. Therefore, there is a problem in that large influences cannot be easily induced on the flying characteristics of the golf ball.
On the other hand, for several seconds after the impact applied to the golf ball, the golf ball flies at a high speed due to strong repulsive power, so that the Reynolds number is increased and the drag coefficient becomes low. After the peak where the driving force of the golf ball becomes weak, the drag coefficient is rapidly increased, and the influence of gravity is dominant, so that the golf ball is landed on the ground with a short staying time.
Therefore, in order to increase the total flying distance of the golf ball, it is important to increase the staying time in the low speed range from the peak to the landing position in the trajectory of the golf ball. Particularly, increasing the staying time in the vicinity of the peak is the most important factor of increasing the flying distance.
Accordingly, in order to increase the flying distance in the low speed range, it is preferable that the diameter of the circular dimple be large. However, as described above, if the diameter of the circular dimple is increased, the golf ball is too greatly influenced by wind, so that the directionality of the golf ball becomes bad. Therefore, the golf ball cannot be landed at the position which is desired by a golfer, but it is landed on an unexpected position. In other words, the golf ball becomes a golf ball having bad controllability.
Recently, in order to overcome these problems, a medium-trajectory golf ball has been used, where an appropriate combination of the circular dimples having a large diameter and having a small diameter is used, so that the directionality and the flying distance are somewhat improved. However, there is a problem in that satisfactory effects cannot be obtained with respect to the directionality and the flying distance.
Korean Patent Laid-Open Publication No. 10-2000-0007178 (published on Feb. 7, 2000) describes a golf ball having dimples arranged by separating the surface of a sphere into a sphere shaped polyhedron for maintaining balance of a resistance.
Korean Patent Laid-Open Publication No. 10-2009-0035130 (published on Apr. 9, 2009) describes a dimple internal structure of a golf ball comprising a plurality of dimples of a concave type formed in a peripheral part of the golf ball and a protrusion portion or a groove portion selectively formed in the lateral part of the whole or partial dimple.